I think that we've basically decided on something like this:
A tunnel ought to be built with hard work, know-how, and the occasional bracing timber. If you try shoring it up with magic, bad things happen. Walls collapse and the entire tunnel gets de-stabilzed. And you never know when it'll be the wrong phase of the moon and the whole thing'll come down.
Like Dwarves. They'd dig out a mountain, and if the stone wouldn't hold, they'd just slap a spell on it. But the only way to keep it stable was to keep packing magic on, and eventually they couldn't hold it together, and they'd leave.
You go in an abandoned Dwarfholt now, and it's huge, shafts, galleries, absolutely rotten with magic. You have to go roped together in the dark, and you can't make a noise, or the whole mountain will come down.
Digger: Page 14Both time-bomb and chaos involved to a greater or lesser extent, for maximum FUN. For Runic/arcane magic anyway, since it's mysterious after all. What could be the kicker is that the only way to make say a Legendary Dining Hall is to do something like this ... beyond Gothic architecture (as in very very few supports for the roof, beyond the upper range of the physically possible that Gothic architecture explores during the middle ages) or maybe the Nobles demand their room's Engravings all be in Glowing Rues but "rune pollution" / side effects make the populace suffer for the Nobles' decadence.
Diety-based "pirest" (non-Christian for more FUN! - more extreme costs/consequences) or "demonic" magic would have prearranged/non-chaotic costs (all those tales where the God curses the mortal or the Demon makes a Deal, etc. all of the consequences of that spell are stated but half-truth deception can still happen of course).
I think Toady One has already figured this out ... it seems pretty self-explanatory and perhaps even common knowledge about how to write Epics/Fantasy and all that.
And nice Beowulf reference for the person a page or two back, but it would have been more helpful if you talked more about the Magic in that story and the associated Costs/chaos the magic users had to presumably deal with and the FUNawesome of that instead of how awesome a Warrior Beowulf was.
By the way, did you know that the comic Digger that I referenced above combines geological timescales of the world being billions of years old with magical/diety shenanigans going on all of that time so that digging in the ground is extremely FUN with all of the ridiculous numbers of old/dead magics/dieties around, though surprisingly the world hasn't turned into one giant magic/diety landfil (I like to think that some Extremophile organisms actually feed off of unprotected magical refuse so that it is defused/recycled but that is not canon in the story). I'm mentioning this because it's clear that the Age of Myths in DF is currently either post-apocalyptic or "pre-seeded with danger" considering where the Funhouses of Clowns deep in the Mountains must have come from, so the situation could possibly be similar to the Digger comic's.